1. Field of the Invention
This invention relates generally to automotive vehicles in which an engine delivers torque to driven wheels via a transmission having a clutch whose engagement pressure is controlled by a control signal. More specifically it relates to improvements in transmission clutch engagement pressure control during conditions when hydraulic fluid that interfaces driving and driven elements of the clutch is relatively more viscous, such as during vehicle warm-up.
2. Background Information
Certain vehicle transmissions employ "wet" clutches in which hydraulic fluid interfaces driving and driven clutch members. It has been observed that engine torque requirements can be significantly larger when the hydraulic fluid is relatively more viscous, such as at initial clutch engagement after engine starting. Hydraulic fluid is generally more viscous at lower temperatures (at and below 10.degree.C.-20.degree.C. for example), and so viscous effects may become more pronounced at lower temperatures. As temperature increases, hydraulic fluid becomes less viscous; consequently, viscous effects on a wet clutch tend to dissipate with increasing temperature. In extreme conditions, viscous effects may sufficiently influence torque production that engagement of a wet clutch can cause noticeably poor shifting and engagement, and perhaps even engine stalling.
Several approaches to the problem of compensating viscosity torque have modeled torque production in order to deduce desired graph plots of torque and clutch engagement pressure during clutch engagement. Results can be stored in the form of pressure vs. torque curves parameterized by variables such as temperature and initial change in rotational speed. However such approaches may be deemed somewhat complex and still subject to some degree of uncertainty due to unit-to-unit variability.
A preliminary novelty search in connection within this invention developed the following U.S. Pat. Nos. 4,509,124; 4,779,489; 5,024,125; 5,050,451; 5,058,014; 5,216,606; 5,307,269; 5,558,597; 5,692,911; and 5,743,826. Also developed were Japan patent publication Nos: 40103515153 (February 1989) and 405272555 (October 1993).